1. Field of the Invention
The present invention relates to a low noise block down converter (hereinafter referred to as an LNB) in use for satellite broadcasting, satellite communications and the like, or an LNB substrate unit incorporated therein.
2. Description of the Background Art
An LNB is a device that performs low noise amplification on received broadband signals having a plurality of channels, while converting them to a lower frequency band in batches.
Presently, to deal with an increased diversity in services such as multi-channel satellite broadcasting, one LNB receives a plurality of microwaves or one LNB is connected to a tuner via a plurality of signal input terminals. Such an LNB may have a complicated circuit structure, causing difficulties in structuring the circuit on one double-sided substrate (double-layer substrate). In a conventional LNB, therefore, signal and power supply lines are tied by joint pins and the like, enabling the use of more than one double-sided substrate. However, the use of more than one double-sided substrate results in a structure with signal and power supply lines that are tied by joint pins, resulting in a bigger, heavier LNB and a more complicated manufacturing process.
One of the solutions to this is to structure an LNB using a multi-layer substrate. A multi-layer substrate is manufactured by stacking double layer substrates and bonding them with an adhesive that serves as a dielectric layer.
Referring to FIG. 31, an LNB four-layer substrate 100 is placed on a chassis 111. LNB four-layer substrate 100 is constructed of a waveguide aperture 113, a probe 114, an antenna pattern 115, first to third ground conductive layers 116–118 and dielectric layers 131–133. Chassis 111 is connected to a waveguide 121, and waveguide aperture 113 communicating with waveguide 121 is formed in LNB four-layer substrate 100. Probe 114 protrudes from LNB four-layer substrate 100 and is located in waveguide aperture 113.
In LNB four-layer substrate 100, antenna pattern 115 is formed of the topmost conductive layer. First to third ground conductive layers 116–118 are formed of the second, third and bottommost conductive layers, respectively, when counted from top to bottom. Dielectric layers 131–133 are provided in between, sandwiched by antenna pattern 115 and first to third ground conductive layers 116–118.
First to third ground conductive layers 116–118 are electrically connected with each other via a connecting hole (not shown). Thus, first to third ground conductive layers 116–118 are at the same electric potential as chassis 111 that is at ground potential. The levels in which first to third ground conductive layers 116–118 are provided are entirely or partially constructed of conductor.
In a conventional LNB four-layer substrate 100 with the above structure, electric wave signals that have been carried along waveguide 121 are introduced into waveguide aperture 113, transmitted through probe 114 to be input into antenna pattern 115.
However, in a conventional LNB multi-layer substrate, the ground conductive layers located within are electrically separated from the housing that steadies the substrate. This tends to effect a loss of wave energy during the passage of the waves, especially when operating at high frequencies. Such deterioration in the passage property presents a problem when a multi-layer substrate is employed instead of a double-sided substrate.
Specifically, in LNB four-layer substrate 100, first ground conductive layer 116 is electrically connected to chassis 111 (ground potential) via second and third ground conductive layers 117, 118. Thus, first ground conductive layer 116 electrically interacts with second and third ground conductive layers 117, 118 and thus cannot easily be maintained at ground potential. Similarly, second ground conductive layer 117 may not easily be maintained at ground potential. This results in a problem of deterioration in the passage property of electric wave signals.